The present invention relates to a process for the preparation of nitric esters of monohydric alcohols.
The cetane number (shortly referred to as CN) is a characteristic analogous to the octane number for the ignition performance of a diesel fuel wherein the shorter the time between the moment the fuel enters the cylinder of the engine and the ignition (ignition delay), the higher the value of the cetane number (cf. Römpp Chemielexikon, 10th Edition, Georg Thieme Verlag Stuttgart/New York, under headword: “Cetan-Zahl”). For many years already nitric esters of monohydric alcohols as e.g. amyl nitrate, hexyl nitrate, octyl nitrate and their isomers, as e.g. 2-ethylhexyl nitrate, have been used in the function of cetane number improvers for diesel fuels.
Though the mononitrates of these alcohols themselves are not listed as explosives and though they are quite stable, their preparation involves some risks. Sometimes the spent acids from the nitration are not stable and may decompose in an uncontrolled way. Apart from that, during nitration vigorous oxidative decompositions of the product, combined with a so called fume-off, or even an explosion may occur (cf. e.g. Health Hazard Evaluation Report No. HETA 82-285-1339, in Chem. Abstracts 102, 190181 (1985)).
Therefore, various efforts have been made to minimize or eliminate these risks.
For instance, U.S. Pat. No. 2,768,964 discloses a continuous and isothermal esterification of monohydric alcohols with mixed acids (from sulfuric acid and nitric acid) with a water content of 30 to 50% in the presence of urea (1 to 10%) at temperatures of 65 to 110° C. in vacuum. The resulting nitric esters are removed from the reaction mixture by distillation. This method shall prevent an uncontrolled oxidative decomposition of the product caused by the omnipresent nitrose oxides in the reaction mixture.
But this method is rather laborious and with long-chain alcohols such as 1-octanol it produces yields of no more than 52%. Moreover, the high water content in the nitrating acids brings about instable acids.
If the esterification is carried out in a stirred tank reactor or a stirred tank reactor cascade in a continuous way under isothermal conditions and without using urea, it is common in the state of the art to work with mixed acids from nitric acid and sulfuric acid having a water content of 0 to 14% and at temperatures as low as possible, i.e. between −15° C. and maximally 20° C., preferably below 10° C., in the presence of an excess of ca. 5% nitric acid, related to the alcohol to be nitrated. (cf. U.S. Pat. Nos. 2,618,650, 2,734,910 and 4,479,905).
In doing so, the weight ratio of water to sulfuric acid in the final spent acid should not exceed 0.35 in order to prevent the risk of a “fume-off” (cf. U.S. Pat. No. 2,734,910).
Apart from that, the residence times of the reaction mixture in the reactors should be as short as possible, particularly between 0.6 and 15 minutes, preferably between 3 and 6 minutes, to prevent an accumulation of side products resulting from oxidative side reactions.
In the state of the art only a combination of the parameters mentioned above, i.e. low temperatures during nitration, short residence times of the reaction mixture in the reactors and a final spent acid with a weight ratio of water to sulfuric acid below 0.35, allows a comparatively secure, continuous isothermal esterification of primary and secondary alcohols with nitric acid.
These methods, as well, are laborious and require a high degree of monitoring.
Surprisingly it was discovered that the problem described above can be solved if the process is carried out in such a way that the monohydric alcohols are continuously and under adiabatic reaction conditions converted with a mixed acid (a mixture of nitric and sulfuric acid).
Hence, the present invention suggests a method according to the claims and specification. Other advantageous embodiments are described and made the subject of the respective dependent claims.